1. The Field of the Invention
The present invention is related to modular heating apparatus and methods for bringing steel to a consistent temperature, particularly for use in connection with steel rolling processes. More particularly, the present invention is related to heating modules which combine induction heating coils and gas holding zones in order to bring steel being processed to consistent temperatures in the core of the steel and on the surface of the steel.
2. Technical Background
There are numerous methods of processing and casting steel. Traditionally, steel was cast into ingots which were later molded or processing into the desired shape or product. The process of casting ingots is now frequently replaced by a process know as continuous casting. In continuous casting processes molten steel is cast directly into a sheet, slab, or other shape. Using continuous casting it is possible to directly form a slab of steel of the desired shape and size without first preparing an ingot. Continuous casting, therefore, is often more simple and economical than the conventional ingot processes.
In conventional continuous casting processes a slab of steel from about 150 mm to 300 mm thick and about 3,000 mm wide is formed. These slabs are then cut into pieces of varying lengths for further processing. Once the slabs are cut into pieces of the desired length they are usually heated and then rolled further in order to produce the desired final product. To produce a flat rolled steel strip, the slabs are generally reheated and then passed through one or more hot rolling roughing millstands. The slabs are then passed through one or more hot rolling millstands in order to further shape the slab. If desired, the product is then passed through reducing and finishing cold rolling millstands to produce a final product.
During the rolling process, heating the slabs to the desired temperature is a difficult and often tedious operation. As the steel slab exits the continuous caster, it is cooled sufficiently that it solidifies to the point that at the very least the exterior surface of the slab is a solid. The interior of the slab may be hotter than the exterior, and may even be close to the melting point. It is then necessary to stabilize the temperature of the slab prior to roller. The ultimate objective is to produce a slab which has a consistent, high temperature throughout, but which is still in solid form. Therefore, as the slab continues through the process in preparation for rolling, the center of the slab cools somewhat, while the exterior surface absorbs the heat applied. This temperature differential is, however, undesirable in that rolling requires a consistent temperature throughout the slab.
In order to produce a slab that has consistent temperature throughout, it is generally necessary to heat the exterior of the slab and then allow the heat to "soak" into the interior. This is usually a multiple step process. Heat is applied by one of several different methods and then allow to soak into the slab. Once this has been accomplished, heat is again applied and once again allowed to soak into the slab. After several applications of heat and several soaking periods it is the process objective that the slab will be at a consistent high temperature, but still below the melting point. Thus, rolling and finishing of the slab can occur in an effective and efficient manner.
Various methods are used during the heating and soaking process described above. One conventional method is to employ a series of induction heating coils to heat the steel slab. Induction heating coils are large coils that encompass the steel slab. An electrical current is passed through the induction coil as the slab is passed through the interior of the coil. A heat producing electrical current is induced in the steel slab. Induction heating coils are widely used and well known to those of skill in the art. As with the general process described above, the heat induced into the slab is then allow to soak into the core of the slab prior to repeating the processing using the next induction coil in the processing line.
The use of induction heating coils, while producing large increases in temperature in the steel slab, presents several problems and limitations as well. Such coils consume large quantities of expensive electrical energy. In a typical conventional induction heating line, it is not unusual to find seven (7) or more coils used to heat the slab to the desired temperature. This series of induction coils consumes large quantities of electricity which substantially increases the cost of the final steel product.
Induction coils also have the limitation of heating the exterior shell of the steel slab. Indeed, it is necessary to careful control the operation of the induction coils in order to make certain that the exterior shell of the slab is not liquified during heating. Thus, induction coils necessitate the use of substantial soaking periods during processing. During the soaking periods the overall temperature of the steel slab will drop substantially prior to application of heat by the next induction coil. Thus, the energy lost during the process is substantial, which at the same time substantially increases the cost of the process.
An alternative heating means is a gas fired furnace or gas heating zone. Gas fired furnaces of this type are also well known in the art. It is possible to roll the steel slabs through a gas fired furnace in order to increase the temperature of the slab. Gas fired furnaces have the advantage of operating on a relatively inexpensive fuel, namely natural gas. Gas fired furnaces have a number of limitations as well. For example, gas fired furnaces provide a more gradual heating of the slab in that they are unable to administer the high levels of energy provided by induction. Thus, gas heating zones must usually be of substantial length in order to achieve the necessary heating.
Because of the length of the typical gas heating zone, it is necessary to provide rollers within the heating zone to support and transport the steel slab. It will be appreciated that temperatures in the 2300.degree. F. range are often reached in the gas heating zone. Thus, it is necessary to employ rollers that can operate undamaged at these high temperatures. In order to survive temperatures in excess of 2300.degree. F. it is generally necessary to use extremely expensive cobalt rollers. These are to be contrasted with less expensive nickel alloy rollers that can be used if temperatures in the 2200.degree. F. ranges are encountered. The difference in cost between the nickel alloy and cobalt rollers is enormous. It will further be appreciated that if it were possible to reduce operating temperatures for the rollers to significantly below the 2200.degree. F., it would be possible to use even less expensive rollers in the gas heating zone. However, this is difficult to achieve in view of the necessity of heating the steel slabs to consistent high temperatures in preparation for rolling and finishing.
A further problem encountered with conventional gas heating zones is maintenance and cleaning. Generally it is necessary to shut down the gas heating zone and allow it to cool to sufficiently that maintenance personnel and enter the gas zones and clean accumulated waste materials. Scale and other debris gradually accumulate in the gas zones and require removal at periodic intervals. Shutting down the gas heating zones generally requires shutting down the entire steel production line. This is extremely expensive and wasteful, particularly in view of the economics of steel production.
Therefore, what is needed in the art are improved methods and apparatus for heating steel slabs to a consistent temperature following casting and before rolling. In that regard, it would be a significant advancement in the art to provide such methods and apparatus which took advantage of the benefits of both gas heating and heating using induction coils. It would be a significant advantage to provide induction heating while significantly reducing the number of induction coils and the corresponding use of electrical energy.
It would also be a significant advancement in the art to provide such methods and apparatus that included gas heating zones, but which overcame some of the significant limitations encountered in the use of conventional gas heating zones. In particular, it would be a significant advancement in the art to provide gas heating zones which were easily cleaned and maintained without the necessity of entirely closing the steel processing line. It would also be a significant advancement in the art to provide gas heating zone which were able to heat steel to high temperatures, even to temperatures in excess of 2300.degree. F., without the necessity of using extemely expensive rollers such as cobalt rollers.
Such methods and apparatus are disclosed and claimed herein.